Equalization system

ABSTRACT

A loudspeaker system which includes a loudspeaker and a drive, the loudspeaker being in an enclosure having a Q of 0.5 or less at a frequency well within the audio band, resulting in rapid rolloff from a frequency well above resonance to about 20 Hz, where cut-off is provided for. The drive has voltage compensation for the rolloff of the speaker, and the latter is directly driven by an amplifier which it sees as a voltage source. Cascaded environmental equalizing circuits are provided, which provide 0, + OR - 2 and + OR - 4 db of mid-frequency (4KC) and of high frequency (20KC) equalization, additional to the low frequency voltage equalization, and the latter is adjustable. Provision is made for delayed application of audio signal to the loudspeakers in the system until voltages have stabilized, to avoid high level transient impulses to the speaker.

Waited States Patent 1191 @orderman 5]March 20, 1973 EQUALIZATION SYSTEMPrimary Examiner-J. D. Miller [75] Inventor: Sidney A. Corderman,Binghamton, Assistant Exammer' Hrvey Fendelman Attorney-Hyman Hurvrtz[73] Assignee: Nlcintosh Laboratory, lnc., [57] ABSTRACT Bmghamton, NY.1 A oudspeaker system which includes a loudspeaker [22] F'led: June 1971and a drive, the loudspeaker being in an enclosure [21] Appl. No.:151,123 having a Q of 0.5 or less at a frequency well within the audioband, resulting in rapid rolloff from a frequency [52] U S u 317/16317/36 TD 317/31 well above resonance to about 20 Hz, where cut-off is/l 41 307/100 337/141 provided for. The drive has voltage compensationfor 51 11111.0. .11011147/12; of the W f lane is directly [58] Field ofSearch ..317/1415 31 36 TD, 16' by amphfier whlch sees as a "wage307/130, 1 100, source. Cascaded environmental equalizing circuits areprovided, which provide 0, i 2 and i 4 db of mid- [56 References Citedfrequency (4KC) and of high frequency (ZOKC) equalization, additional tothe low frequency voltage UNITED STATES PATENTS equalization, and thelatter is adjustable. Provision is 2,867,754 1/1959 OBleness ..317/141smade for delayed application of audio Signal to the 3,309,583 3/1967Ziller ..317/31 ou speakers in the system until voltages have stabil-,146 8/1934 Rovere ...-3l7/16 ized, to avoid high level transientimpulses to the 3,320,493 5/ l 967 Culbertson ..3 17/3 l speaker3,144,568 8/1964 Silliman ..3l7/l4l S 4 Claims, 7 Drawing Figures 1 016155 EGFPETCTEcUE "'2 l l l I l l l PATENTEDHARZOIQH SHEET 4 BF 4 LULFREQUENCY CONTROL, RESPONSE mu FQmuENcY m Hana MlD FREQUENCY CUNTRDL.RESPONSE.

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\nnu FREQUENCY \N HERTZ m8 6420 4 m o m mm 2 mwz wwm INVENTDE SIDNEYF\.CDRDERMRN w M ATTDQNET EQUALIZATION SYSTEM BACKGROUND Thisapplication is related to an application for U. S. Pat., Ser. No.873,264, filed Nov. 3, 1969, which is assigned to a common assigneeherewith. That application presents in block form the present system,minus some of its refinements, such as provision for environmentalequalization and protection of speakers against transients. Thisapplication discloses the circuitry appropriate to the priorapplication.

In a loudspeaker, the relationship between cone displacement and drivingforce that moves the cone, must be a straight line, if natural sound isto ensue. Any departure from linearity results in departure from perfectreproduction. But even worse, any departure from linearity producesdistortion, which in terms of percentage of the original signal is afunction of nonlinearity. Further to avoid distortion, the cone of thespeaker must move as a piston at all frequencies, i.e., the entiresurface of the cone must move the same distance in the same time. If onepart of the cone moves differently from another part, called breakup,distor tion occurs.

Heretofore, perfect piston cones have been avoided, because such conesmust be stiff, and to achieve stiffness involves weight of cone. Weightof cone dictates frequency range of the cone, i.e., as weight goes upstiffness increases, but frequency range and speaker efficiency inconverting electrical energy to acoustic energy, decreases. In the lowfrequency speaker of the present system, the cone and drive coilassembly have an effective moving mass of 55 grams. Frequency of driveof a loudspeaker is proportional to acceleration of its cone and coil.For example, at 60 Hz, with a l2 inch loudspeaker producing l acousticwatt, maximum acceleration is 93 grams, which requires great electricalpower. In the present system, the low frequency speaker cuts off at 250Hz.

Another problem faced in designing loudspeakers, is that of radiationpatterns. If the wave length to be radiated is long in relation toloudspeaker diameter, intensity of sound is the same all around thespeaker, i.e., front, back, sides, above and below. Wave lengthdecreases from 55 feet at Hz to A inch at 20 KHz. To achieve widelydispersed radiation patterns loudness is lost, and enough speakers mustbe employed so that the shortest wave length radiated is at leastcomparable to loudspeaker diameter.

As wave length shortens, a speaker becomes more and more directional,until its pattern breaks up into major and minor lobes. For example, aspitch increases from 1500 Hz to 3000 Hz a listener 300 off axis wouldhear sound of greatly reduced intensity. The brain localizes stereosounds by means of cues, which include relative intensities from thestereo speakers. As pitch varied then, the listener who is off-axiswould obtain the impression of a moving sound source, because theradiation pattern changes. Changes of intensity of the order of db canoccur. The problem is avoided by employing many loudspeakers ofdiffering diameters. A 12 inch speaker may be limited to 300 Hz, asmaller speaker extend the range to 1500 Hz, another to 7000 Hz, etc.Limiting frequency range per speaker allows piston-like operation, sothat better sound imaging and better stereo imaging, and lessdistortion, all require limiting individual speakers to sub-bands of thetotal audio spectrum, and the improvement accrues throughout a room, andnot only axially of the speakers. Stereo imaging pertains to separationof instruments and their definition and localization.

Having designed a loudspeaker which is linear with frequency down to 20Hz, and which radiates spherically, it is found that location of theloudspeaker in a room causes power output to vary by as much as 9 db.For example, if a loudspeaker is suspended mid-room it radiatesspherically, but if it is on the floor it does not, due to reflectionsfrom the floor. Such reflections double loudness. Location against awall again doubles loudness, and location in a corner adds 3 db.Provision is made in the present system to adjust bass compensation forroom location of the loudspeaker. Further provision is made, at mid andtreble frequencies, for

the effects of drapes, reflecting walls and floors, rugs and the like,by enabling 2% and i 5 db of gain or loss in these ranges.

SUMMARY A loudspeaker system designed for (1) flat bass response to 20Hz, (2) wide angle radiation patterns at all frequencies, (3)piston-like speaker cones, (4) essentially no intermodulationdistortion, (5) compensation for room location, and room reflectivity,for midrange and treble frequencies, (6) adequate cone travel (V4 to 1inch) in the bass to move the required volume of air for an effective 10inch radiator, (7) high efficiency speaker with overdamping at afrequency within the audio band, achieved by electrical and acousticdamping in terms of magnet and coil design and of small enclosure volumeper speaker.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of aloudspeaker-cross over arrangement according to the present invention;

FIG. 2 is a block diagram of a 'drive system for the loudspeakers;

FIG. 3 is a schematic circuit diagram of the system according to FIG. 2;

FIG. 4 is a schematic circuit diagram of a power supply for the systemof FIG. 2, including a time delay circuit; and

FIGS. 5 7 are, respectively, curves of available responses in DB forvarious settings of equalization switches, in the system of FIGS. 2 4.

DETAILED DISCLOSURE All the loudspeakers of the present system areconnected across lines 10, I l, and speakers of each size are associatedwith appropriate cross-overs. LSl-LS4, inclusive, are 12 inchloudspeakers, connected in parallel pairs. A filter coil L, is connectedbetween line 10 and the speakers LSl-LS4, and a series CR by-pass forhigh frequencies across the speakers. The DC resistance of L1 isimportant, in that the power amplifier connected between lines 10 and 11must have a low internal resistance, in order to appear as a voltagesource to its load, and the DC resistance of L1 must be ve rysmall, atleast by a factor of 1:10, relative to the DC resistance of the speaker,or about 0.8 ohms. Similarly, its reactance must be small relatively tospeaker coil reactance in the pass band. The speakers are labelled as tosize, and the frequency bands they radiate, and cross-over filtercomponents are identified, all on the drawings. The requirement that theloudspeaker be directly connected to a low impedance amplifier appliesstringently only to the speakers LSL-LS4, inclusive, since for theremaining speakers there is no necessity to move large volumes of air,and power is much smaller than at low frequencies.

Since the Left and Right sides of the present system are identical, onlyone side is illustrated and described.

In FIG. 2, audio signal is applied to a terminal 20, whence it proceedsto input attenuator 21, and when at a desired level to a sub-sonic highpass filter 22 having a sharp cut-off at 20 Hz. The filter 22 is of theactive type including an amplifier 22a and a feedback path 22b. Theoutput of filter 22a proceeds to a mid-frequency equalization circuit23, which boosts or lowers Signal level over a band centered at 4 KC.The circuit 23 provides five positions; flat, 2db, 4db, 2 db and 4 db.

The circuit 23 feeds in cascade to a high'frequency equalization circuit24 providing variable rolloff of db, 2db, 4db, 2db and 4db up to 20 KC.

The circuit 24 feeds a low frequency equalization circuit 25,represented by an active filter composed of an amplifier 25a and afeedback network 25b. The circuit 25 in turn feeds an output terminal 26via a turn on relay 27, which provides delay in applying voltage.

The power supply of the system includes a plug 30, a fuse 31 in serieswith the plug 30, a power transformer 32, which drives a 75 V powersupply and regulator 33, which in turn supplies all amplifier stages ofthe system via lead 34, and also supplies power to turn on delay system27.

FIG. 3

In FIG. 3 terminals 40, represent possible input terminals, as labelled,which can be selected by switch 41 and applied to attenuators 42. In theposition illustrated all attenuator resistances R1-R6 are shorted, butas bars 42 move to the right attenuation increases, values ofattenuation in db being illustrated. The attenuated output signal forthe left channel is ignored as duplicative.

The signal proceeds to active high pass filter 22, designed to havesharp cut-off at 20 Hz. Design of filter 22 is conventional, and itsoutput at low impedance level is taken from emitter loads 50, forapplication to variable step attenuator 23, which provides a range ofequalization curves, as illustrated in detail in FIG. 6. The output ofattenuator 23 proceeds to attenuator 24, which provides a range ofattenuator curves as illustrated in FIG. 7, and the output of equalizer24 is applied to active filter 25a having a variable feedback network25b, which establishes the five attenuation curves of FIG. 5. Inamplifier 25, signal is applied to the base of which is collectorcoupled to the base of Q7. The latter is emitter coupled to the base of09, having a collector load 54. The output of 09 is the left output ofthe equalizer of FIG. 3. The amplifier 25 includes a feedback pathadjustable by means of switch 55, the path extending from the collectorof 09 via lead 56, the various KC circuits of feedback path 25 b, andback to the emitter of 05 via lead 57. The filter 25 is per se wellknown, and therefore does not merit detailed discussion.

FIG. 4

In FIG. 4, the transformer 32 supplies AC voltage to full wave rectifier60, which provides dc output voltage at lead 61. The latter is connectedvia resistances 62 and 63 to large capacitor 64. Transistor 0101 isnormally non-conductive, absent voltage, since both its emitter and baseare grounded. But as the capacitor 64 charges, the voltage of the baseof 0101 rises and the transistor becomes conductive supplying voltage tooutput lead 65 at V, and via a voltage divider 66 to leads 67 at 10 V.

The voltage at the junction of resistances 62, 63, is applied to thebase of transistor 0102, which has a grounded emitter, via Zener diodeD104. The collector of 0102 is connected in series with a relay coil K1and is supplied with voltage from a rectifier power supply 71. The relaycoil K1 actuates switch contacts 73, 74, which normally are closed andground the inputs I to the power amplifiers of the system. When ac poweris applied to the system the collector of 0102 goes positiveimmediately. However the base of 0102 is isolated by the Zener diodeD104 and the relay K1 remains unenergized. When the voltage at thejunction of resistances 62, 63 rises to a sufficient value, as capacitor64 charges, Zener D104 breaks down, transistor 0102 becomes conductive,and coil K1 energizes, pulling ground off output leads and permittingthe power amplifier of the system to operate. The time constant involvedis about 4 seconds. If power fails, the contacts 73, 74, immediatelyclose, and when power comes on again, again require 4 seconds to open.Thereby, transient sounds and impulses applied to the loudspeakers arereduced. If desired, leads I can proceed directly to loudspeakers, as analternative system, since contacts 73, 74, are connected to ground viaresistances 77, 78, which supply loads for the power amplifier, untilconditions stabilize after application of power to the system. In thepresent system, the circuit of FIG. 3 is called an environmentalequalizer, and it feeds a pre-amplifier, power amplifier andloudspeaker, and constitutes a unit not present in prior art highfidelity systems. The primary function of this system is to compensatefor speaker placement and room environmental conditions.

What I claim is:

1. A protective circuit for an amplifier having an output including arelay having a coil and contacts normally connected to ground saidoutput when said relay is unenergized, comprising a power supplyincluding as output a source of DC voltage, a timing circuit comprisinga resistance and a capacitor connected in series in the order recitedbetween said source of voltage and ground, a transistor having acollector connected to energize said coil and having a grounded emitter,a Zener diode connected at one side to the base of said transistor andat the other to an ungrounded side of said capacitor, a second source ofvoltage, and means providing a series circuit including said secondsource of voltage, said relay coil and said collector, said timingcircuit having a time constant of several seconds, said contacts whenopen being arranged to remove said ground from said output.

2. A protective circuit for an amplifier output load and a delay circuitfor application of voltage to said amplifier comprising a. a source ofpositive DC voltage,

b. a first PNP transistor having a first collector, first base and firstemitter,

c. a resistance connecting said first emitter to ground,

d. a lead connected to said resistance for deriving said voltagetherefrom,

e. a timing capacitor connected between said first base and ground,

f. two timing resistances in series connected between said source of dcvoltage and said first base,

g. a second PNP transistor having a second emitter, a

second base and a second collector,

h. a Zener diode connected between the junction of said two timingresistances and said second base,

i. means grounding said second emitter,

j. a voltage source,

k. a relay having a coil connected between said voltage source and saidsecond collector,

1. said relay having a stationary contact connected to said load,

m. said relay including a movable contact normally connected to groundand contacting said stationary contact and responsive to energization ofsaid coil for pulling away from said stationary contact.

3. A protective circuit for an audio amplifier, comprising a powersupply for said audio amplifier, said power supply having an outputterminal, an RC timing circuit connected between output terminal andground, said RC timing circuit having a time constant of at least asecond and having a series timing resistance and a timing capacitor,said timing capacitor having a grounded and an ungrounded terminal, apower terminal for said amplifier, a normally non-conductive transistorswitch connected in series between said output terminal and said powerterminal, means responsive to achievement of a first predeterminedvoltage across said timing capacitor for rendering said transistorswitch conductive, a relay having a coil and a movable and stationarycontact, a load connected to said stationary contact, said movablecontact normally being grounded and normally contacting said stationarycontact, a further transistor switch responsive to said voltage acrosssaid timing capacitor for energizing said coil and separating saidcontacts on attainment of a second predetermined voltage across saidtiming capacitor.

4. The combination according to claim 3, wherein said load is aloudspeaker connected to said amplifier.

1. A protective circuit for an amplifier having an output including arelay having a coil and contacts normally connected to ground saidoutput when said relay is unenergized, comprising a power supplyincluding as output a source of DC voltage, a timing circuit comprisinga resistance and a capacitor connected in series in the order recitedbetween said source of voltage and ground, a transistor having acollector connected to energize said coil and having a grounded emitter,a Zener diode connected at one side to the base of said transistor andat the other to an ungrounded side of said capacitor, a second source ofvoltage, and means providing a series circuit including said secondsource of voltage, said relay coil and said collector, said timingcircuit having a time constant of several seconds, said contacts whenopen being arranged to remove said ground from said output.
 2. Aprotective circuit for an amplifier output load and a delay circuit forapplication of voltage to said amplifier comprising a. a source ofpositive DC voltage, b. a first PNP transistor having a first collector,first base and first emitter, c. a resistance connecting said firstemitter to ground, d. a lead connected to said resistance for derivingsaid voltage therefrom, e. a timing capacitor connected between saidfirst base and ground, f. two timing resistances in series connectedbetween said source of dc voltage and said first base, g. a second PNPtransistor having a second emitter, a second base and a secondcollector, h. a Zener diode connected between the junction of said twotiming resistances and said second base, i. means grounding said secondemitter, j. a voltage source, k. a relay having a coil connected betweensaid voltage source and said second collector, l. said relay having astationary contact connected to said load, m. said relay including amovable contact normally connected to ground and contacting saidstationary contact and responsive to energization of said coil forpulling away from said stationary contact.
 3. A protective circuit foran audio amplifier, comprising a power supply for said audio amplifier,said power supply having an output terminal, an RC timing circuitconnected between output terminal and ground, said RC timing circuithaving a time constant of at least a second and having a series timingresistance and a timing capacitor, said timing capacitor having agrounded and an ungrounded terminal, a power terminal for saidamplifier, a normally non-conductive transistor switch connected inseries between said output terminal and said power terminal, meansresponsive to achievement of a first predetermined voltage across saidtiming capacitor for rendering said transistor switch conductive, arelay having a coil and a movable and stationary contact, a loadconnected to said stationary contact, said movable contact normallybeing grounded and normally contacting said stationary contact, afurther transistor switch responsive to said voltage across said timingcapacitor for energizing said coil and separating said contacts onattainment of a second predetermined voltage across said timingcapacitor.
 4. The combination according to claim 3, wherein said load isa loudspeaker connected to said amplifier.